Field of the Invention
This invention relates to a system for dissolution-purification of Zn-containing materials, and more particularly to a system for purifying Zn-containing materials discharged from various plants of an iron mill etc. by dissolution of the Zn-containing materials through countercurrent contact with a dissolving liquid, to recover zinc carbonate (ZnCO.sub.3)
Zinc oxide (ZnO) used as a vulcanization accelerator for tires, or for pigments, etc. is, generally, prepared by the dry process such as the American method or French method, or the wet process.
In this connection, it is to be noted that Zn-containing materials are discharged from various plants of iron mills etc. such as reduced iron manufacturing plant, a Zn-plating plant, or the like. These Zn-containing materials contain a considerable amount of zinc, and therefore, it is desired to effectively recover Zn from such Zn-containing materials.
If the dry process as mentioned above is applied to the preparation of ZnO from said Zn-containing materials, the resultant products are inevitably contaminated with several-thousand ppm of iron component and lead component originally included in the starting Zn-containing materials. In the case of the wet process, iron component in the Zn-containing materials is also dissolved in an amount of tens of thousands ppm, when the Zn-containing materials are subjected to dissolution in hydrochloric acid.
Even if the products contain several-tens of ppm of iron and/or lead, they may be used for some applications. However, it is desirable, in general, to reduce the iron or lead components and preferably to reduce the iron component to 10ppm or less and the lead content to 1 ppm or less.
The inventors of the present invention have previously proposed, in Japanese Patent Application Laid-Open (Kokai) No. 59-88319, a process for recovering Zn from Zn-containing materials.
This previously proposed process is an improvement of the wet process. This process is based on the fact that zinc is an amphoteric metal. The basic idea of this process is to contact the Zn-containing materials with a liquid to dissolve Zn and let impurities included in the Zn-containing materials be precipitated, and subject the filtrate, in from which the undissolved residue has been removed, to recover zinc carbonate in the form of 2ZnCO.sub.3 3Zn(OH).sub.2 and/or ZnCO.sub.3 3Zn(OH).sub.2.
As described above, hydrochloric acid is used as a solvent in the conventional wet process. The starting material of the conventional wet process is metal powder, so that special treatment of impurities is not needed in the conventional process. However, in the previously proposed inventors' process, the starting materials are not pure Zn metal, but rather, are Zn-containing materials including at least Fe and Pb, such as kiln dust from a reduced iron preparing plant or a discharge from a Zn-plating plant. Therefore, the most important issue of that process is to remove the impurities such as Fe or Pb and to obtain a zinc carbonate containing minimized impurities.
To attain the task as mentioned above, according to the inventor's previous process, the ampholytic property of Zn is utilized. More specifically, a solution mixture of NH.sub.4 OH and (NH.sub.4).sub.2 CO.sub.3 or a solution mixture thereof to which is further added, according to necessity, ammonium thiocyanate (NH.sub.4 SCN) and/or ammonium sulfide (NH.sub.4).sub.n S is employed as a solvent, and the Zn-containing materials are dissolved in such a solution and then subjected to an approprate post-treatment, thereby to effectively remove the impurities such as Fe and Pb.
In the process, however, additional steps such as air oxidation, metal zinc adding, filtration of iron oxides, or ion exchange are needed to minimize the Fe and Pb components in the products, because Fe and Pb are also dissolved in the basic ammonium carbonate solution when the Zn-containing materials are dissolved in the solution. Thus, the process is not economical.
Therefore, it has been desired to provide a process which is capable of minimizing the post-treatment after the dissolution of the Zn-containing materials and further reducing the Fe and Pb contents in the products.
On the other hand, it has been found by the inventors that when the Zn-containing materials as mentioned above are dissolved in the basic ammonium carbonate solution, such an arrangement, where a plurality of simple dissolving tanks or tanks with clarifying portions are simply disposed sequentially, has the following problems.
The Zn-containing dust is reacted with the above-mentioned solution as follows: